The semimembranosus (also semimembranosus muscle, latin: musculus semimembranosus) is a flat, long muscle situated in the posterior region of the thigh.
The semimembranosus is so called because of its membranous tendon of origin.
The semimembranosus originates from the ischial tuberosity.
The semimembranosus muscle inserts onto the medial condyle of the tibia.
The main functions of the semimembranosus are extension of the thigh at the hip joint, and flexion and internal rotation of the knee.
The semimembranosus is innervated by the tibial nerve, a branch of the sciatic nerve, which arises from the sacral plexus.